1. Field of the Invention
This invention relates to an information recording medium which is used for optically or electrically recording, erasing, overwriting and reproducing information, and a method for producing the same.
2. Description of Related Art
The inventors developed 4.7 GB DVD-RAM which is a large capacity rewritable phase-change type information recording medium and can be used as a datafile and an image file. This has been already commercialized.
This 4.7 GB DVD-RAM is disclosed, for example, in Japanese Patent Kokai (Laid-Open) Publication No. 2001-322357. The constitution of DVD-RAM disclosed in this publication is shown in FIG. 10. The information recording medium 31 shown in FIG. 10 has a seven-layer structure where a first dielectric layer 102, a first interface layer 103, a recording layer 4, a second interface layer 105, a second dielectric layer 106, an optical compensation layer 7, and a reflective layer 8 are formed on one surface of a substrate 1 in this order. In this information recording medium, the first dielectric layer exists in a position closer to an incident laser beam than the second dielectric layer. The same relationship exists between the first interface layer and the second interface layer. Thus, in this specification, when the information recording medium contains two or more layers having the same function, “first” “second” “third” . . . is given to the beginning of the name of each layer in the order of the layer which is closer to the incident laser beam.
The first dielectric layer 102 and the second dielectric layer 106 have the function which adjusts an optical path length so as to enhance the optical absorption efficiency of the recording layer 4, and enlarges the difference between the reflectance of crystal phase and the reflectance of amorphous phase so as to enlarge a signal amplitude. ZnS-20 mol % SiO2 (i.e. (SiO2)80(ZnS)20) conventionally used as a material for the dielectric layer is amorphous material. It has low thermal conductivity, is transparent, and has a high refractive index. Moreover, ZnS-20 mol % SiO2 exhibits a high film-forming speed at the time of the film formation, and good mechanical characteristic and moisture resistance. Thus, ZnS-20 mol % SiO2 is an excellent material suitable for forming the dielectric layer.
If the thermal conductivity of the first dielectric layer 102 and the second dielectric layer 106 is low, the heat can diffuse from the recording layer 4 to the reflective layer 8 quickly in the thickness direction when a laser beam enters the recording layer 4, and therefore, the heat is difficult to diffuse in the in-plane direction in the dielectric layers 102 or 106. That is, the recording layer 4 is cooled by the dielectric layer for a shorter time, and therefore, an amorphous mark (record mark) can be easily formed. When a record mark is hard to form, a high peak power is necessary for recording. When a record mark is easy to form, recording can be conducted with a low peak power. When the thermal conductivity of the dielectric layer is low, recording can be conducted with a low peak power, and therefore, the recording sensitivity of the information recording medium becomes higher. On the other hand, when the thermal conductivity of the dielectric layer is high, recording is conducted with a high peak power, and therefore the recording sensitivity of the information recording medium becomes lower. The dielectric layer in the information recording medium exists in a form of such thin film that thermal conductivity cannot be measured accurately. For this reason, the inventors employ the recording sensitivity of the information recording medium as a relative judgment reference for learning the degree of the thermal conductivity of the dielectric layer.
The recording layer 4 is formed using a material containing Ge—Sn—Sb—Te which crystallizes at a high speed. The information recording medium which contains such material as the recording layer 4, not only has excellent initial recording characteristic, but also has excellent archival characteristic and an excellent archival overwrite characteristic. In a phase-change type information recording medium, information is recorded, erased and overwritten by utilizing reversible phase change between crystal phase and amorphous phase of the recording layer 4. When the recording layer 4 is irradiated with a high power (i.e. peak power) laser beam, and then cooled rapidly, the irradiated part turns into an amorphous phase and a record mark is formed. When the recording layer is irradiated with a low power (i.e. bias power) laser beam to raise its temperature and then cooled gradually, the irradiated part turns into a crystal phase and recorded information is erased. By irradiating the recording layer with the laser beam of which power is modulated between the peak power level and the bias power level, it is possible to overwrite new information while erasing information already recorded. Overwrite cycle-ability is expressed with the maximum number which corresponds to repeatable overwrite number on the condition that the jitter value does not cause a problem in a practical use. It can be said that the better overwrite cycle-ability is, the larger this number is. Particularly, an information recording medium for datafiles is expected to have excellent overwrite cycle-ability.
The first interface layer 103 and the second interface layer 105 have the function which prevents a material transfer caused between the first dielectric layer 102 and the recording layer 4, and between the second dielectric layer 106 and the recording layer 4, respectively. The material transfer here means the phenomenon that S of ZnS-20 mol % SiO2 contained in the first and second dielectric layers diffuses into the recording layer while the recording the layer is irradiated with a laser beam and information is repeatedly overwritten. If a lot of S diffuses into the recording layer, a reduction of the reflectance of the recording layer is caused, and overwrite cycle-ability deteriorates. This phenomenon has already been known (See N. Yamada et al. Japanese Journal of Applied Physics Vol.37 (1998) pp.2104-2110). Moreover, Japanese Patent Kokai (Laid-Open) Publication No. 10-275360 and International Publication No. WO 97/34298 disclose that the interface layer which prevents this phenomenon is formed using a nitride containing Ge.
The optical compensation layer 107 adjusts the ratio Ac/Aa where Ac is optical absorptance of the recording layer 4 in a crystalline state, and Aa is optical absorptance Aa of the recording layer 4 in an amorphous state, and serves to suppress distortion of overwritten marks. The reflective layer 8 optically serves to increase the light quantity absorbed by the recording layer 4, and thermally serves to diffuse the heat generated in the recording layer 4 to cool the recording layer quickly and to facilitate amorphization of the recording layer. The reflective layer 8 also serves to protect a multilayered film from the operation environment.
Thus, the information recording medium shown in FIG. 10 ensures excellent overwrite cycle-ability and high reliability with a large capacity of 4.7 GB by using the structure including the seven layers each of which functions as mentioned above, and thereby has been commercialized.
As material suitable for the dielectric layer of the information recording medium, various materials have already been proposed. For example, in Japanese Patent Kokai (Laid-Open) Publication No. 5-109115, it is disclosed that a heat-resistance protective layer is formed from a mixture of a high melting point element with a melting point above 1600K and low alkali glass in an optical information recording medium. In this publication, Nb, Mo, Ta, Ti, Cr, Zr, and Si are mentioned as the element with a high melting point. Further, in this publication, it is disclosed that the low alkali glass essentially consists Of SiO2, BaO, B2O3, or Al2O3.
In Japanese Patent Kokai (Laid-Open) Publication No. 5-159373, it is disclosed that the heat-resistance protective layer is formed from a mixture of at least one compound selected from nitride, carbide, oxide and sulfide with a melting point higher than that of Si, and low alkali glass in an optical information recording medium. In this publication, the carbide, oxide, and sulfide of Nb, Zr, Mo, Ta, Ti, Cr, Si, Zn, and Al, are illustrated as the high melting point compound. Moreover, in the publication, it is disclosed that the low alkali glass essentially consists of SiO2, BaO, B2O3, and Al2O3.
In Japanese Patent Kokai (Laid-Open) Publication No. 8-77604, it is disclosed that a dielectric layer of a read-only information recording medium is formed from the oxide of at least one element selected from the group which consists of Ce, La, Si, In, Al, Ge, Pb, Sn, Bi, Te, Ta, Sc, Y, Ti, Zr, V, Nb, Cr, and W, the sulfide of at least one element selected from the group which consists of Cd, Zn, Ga, In, Sb, Ge, Sn, Pb, and Bi, or selenide and so on.
In Japanese Patent Kokai (Laid-Open) Publication No. 2001-67722, it is disclosed that the first interface control layer and the second interface control layer of an optical recording medium are selected from the nitride, oxide, carbide, and sulfide which contain at least one element selected from the element group consisting of Al, Si, Ti, Co, Ni, Ga, Ge, Sb, Te, In, Au, Ag, Zr, Bi, Pt, Pd, Cd, P, Ca, Sr, Cr, Y, Se, La, and Li.